Double metal cyanide compounds are known to be extremely active catalysts for the polymerization of epoxides, as disclosed for example in U.S. Pat. No. 3,278,457. They have also been employed in copolymerizations of epoxides and other monomers such as cyclic anhydrides and carbon dioxide (U.S. Pat. Nos. 3,438,043 and 4,500,704). Such catalysts have been found to be particularly useful for the preparation of polyether polyols, as taught by U.S. Pat. No. 3,829,505. Polyether polyols produced in this manner have lower levels of unsaturation and hence higher average actual hydroxyl functionality than polyols obtained using conventional base-catalyzed methods. Polyether polyols are commercially employed in a wide variety of applications, including polyurethane foams and elastomers.
In spite of the high activity of the double metal cyanide complex catalysts, which allows such catalysts to be used at relatively low concentrations, the prior art has recognized that it is desirable to remove as much of the catalyst as possible from the polymer following polymerization U.S. Pat. No. 4,355,188 teaches that the residual catalyst catalyzes allophanate formation when a polyether polyol is reacted with an isocyanate and also promotes the formation of undesirable volatile by-products during storage.
However, removal of the catalyst is complicated by its solubility in the polymer. U.S. Pat. No. 3,829,505 (col. 6, lines 17-21) teaches that the double metal cyanide complex catalyst, while initially insoluble in the polymerization medium, becomes very well dispersed if not completely dissolved during the polymerization process. In addition, this reference teaches that dilution of the polymer product in a non-viscous solvent, followed by centrifugation, is ineffective in removing a major proportion of the catalyst. Thus, the methods which have been developed in the prior art to effectively remove double metal cyanide complex catalyst from a polymer all involve some type of chemical treatment of the polymer product.
U.S. Pat. No. 4,344,188 teaches that removal of the residual catalyst may be accomplished by treating the crude polymer with a strong base such as an alkali metal hydroxide or alkali metal dispersion and then passing the polymer through an anionic exchange resin.
U.S. Pat. No. 4.721,818 teaches that catalyst removal is achieved by reacting the polymer with an alkali metal hydride so as to convert the double metal cyanide complex catalyst into an insoluble species which can then be removed by filtration. An adsorbent such as magnesium silicate may be employed to remove any excess alkali metal hydride remaining.